1. Field of the Invention
The present invention relates to a thin-film transistor circuit that uses a crystallized semiconductor thin film having crystal grains of relatively large size, a design method for the thin-film transistor circuit, a design program for the thin-film transistor circuit, a design program recording medium, a design library database, and a display device.
2. Description of the Related Art
Flat-panel display devices have found use in various fields because of their features of thinness, lightness, and low power dissipation. Above all, active-matrix flat-panel display devices have been widely used in OA equipment in which high resolution and high picture quality are required. This type of display device includes a number of thin-film transistors (TFTs) each of which is placed adjacent to a respective one of pixels arranged in the form of a matrix and connected to it as a pixel switching element. With the recent development of multimedia communication technology, a function-intensive flat-panel display device has attracted attention as a display device for next generation personal uses. This flat-panel display device has a structure, called a system on panel, in which diversified peripheral circuits including a driver circuit, a memory circuit, a DA conversion circuit, and a image processing circuit, which are associated with image display, are integrated together with the pixel array.
In order to realize the function-intensive display device, it is required to constitute peripheral circuit components, such as logic circuits, memory elements, analog amplifiers, etc., by thin-film transistors using a semiconductor thin film of high carrier mobility formed on an insulating substrate made of glass or the like. Heretofore, various techniques have been proposed to form such a thin film at low temperatures.
With a recrystallization method using laser light, it is possible to obtain a silicon film having crystal grains of, say, a rectangular shape as a crystallized semiconductor thin film (see, for example, Japanese Unexamined Patent Publications Nos. 2002-237455 and 2003-22969).
Transistors are individually placed in crystal grains of a crystallized semiconductor thin film obtained by the recrystallization method using laser light (see, for example, Japanese Unexamined Patent Publications Nos. 2003-86604, 2003-31497, 2003-124230, 2003-318127, 2003-197521, 2003-197527).
The above-mentioned Patent Publications disclose techniques to obtain crystal grains sufficiently large in size to accommodate the channel regions of thin-film transistors, but do not disclose techniques to properly and efficiently design a thin-film transistor circuit, such as a peripheral circuit, on a crystallized semiconductor thin film having such crystal grains.
In the manufacture of a thin-film transistor circuit, it is required to prepare various photomasks for patterning a crystallized semiconductor thin film and wiring layers. The aforementioned conventional techniques will not be sufficient to prepare practical photomasks.
In preparing photomasks, it is required to determine the arrangements of thin-film transistors and wirings necessary for a thin-film transistor circuit and reflect these arrangements in various photomasks. With a large-scale thin-film transistor circuit, however, laying out all the thin-film transistors and wirings artificially would inevitably increase the design period and the frequency of occurrence of erroneous wirings, resulting in an accelerative increase in design cost.
Conventionally, there exists a design tool which is a computer that simulates a thin-film transistor circuit. The use of this design tool allows the layout of thin-film transistors and wirings to be automated. This layout result allows for the channel regions of thin-film transistors to be placed in portions of poor crystallinity of a crystallized semiconductor thin film. It is therefore difficult to apply the automated layout to photomasks for a thin-film transistor circuit using a crystallized semiconductor thin film.
In the aforementioned Patent Publication No. 2003-31497 or 2003-318127, methods are disclosed which form crystal grains of large area through the use of a phase-shifting laser-based crystallization method. When this method is used in forming an actual thin-film transistor circuit, in order to form crystallized areas corresponding in arrangement to thin-film transistors constituting the transistor circuit, it is required to specially design the pattern of a phase-shifting mask for phase-modulation of laser light. The disclosed contents of the aforementioned known documents are not sufficient to efficiently prepare a practical phase-shifting mask.